Embarassing question... but is it normal to be having erection difficulties at age 26?

For the past month or so.. my erections have been off and on with my girlfriend...
and for the past 3 days I haven't even been able to OBTAIN one..

It doesnt make much sense because i exercise regularly ( 4-5 times a week ), eat well, get enough sleep, and im not stressed out about anything, and my sex drive is sure there! I want it.... but its just not happenin'...

It's starting to take its toll on our sex life and im wondering what the issue might be and if anyone has experienced this before?